The current disclosure relates to the field of digital signal processing. More specifically, the present disclosure relates to a system and method of measuring the instantaneous period of a quasi-periodic signal.
Many monitored signals may be characterized as quasi-periodic signals. These signals exhibit a generally repeating wave form, but at a constantly changing frequency. One such example of a quasi-periodic signal is a returned ultrasound signal exhibiting a Doppler effect. The Doppler effect may be due to the motion of a fetus' beating heart. While ultrasound signals have a variety of applications in medical and other fields, including imaging, diagnostic, and therapeutic applications, one such application is the use of ultrasound signals to monitor fetal heart rate.
In the measurement of fetal heart rate (FHR), it is desirable to be able to determine the instantaneous period of the fetal heart activity at each heart beat. The determination of beat to beat fetal heart rate can have additional diagnostic value in determining the condition of the fetus compared to an average fetal heart rate alone.
One of the problems facing the accurate measurement of instantaneous fetal heart rate is that measurements of instantaneous period are highly susceptible to noise and other artifacts in the ultrasound signal. This issue is commonly addressed by taking the average period over a plurality of intervals in order to filter out some of the noise. This reduces the noise, but decreases the accuracy of the instantaneous heart rate determination as the heart rate measurement is based upon multiple beats. More accurate results may be obtained by shortening the length of the measurement window, but as the number of heart beats used for the determination is reduced, the susceptibility to noise and artifacts increases. Therefore, a balance must be struck when selecting the number of beats over which the signal is averaged to balance the noise reduction and heart rate determination accuracy considerations.